Kusarigama (鎖鎌, くさりがま, "Chain-sickle"), is a traditional Japanese weapon that consists of a kama (the Japanese equivalent of a sickle) on a metal chain (kusari) with a heavy iron weight (fundo) at the end. The kusarigama is said to have developed during the Muromachi period. The art of handling the kusarigama is called kusarigamajutsu. The weapon is used by Yōkai taijiya like Kohaku. The sickle of Kohaku's weapon is made of demon bone, either shaped into, or crafted from a single piece of bone that makes up the handle and blade.
Coincidentally the Kusarigama was a popular ninja weapon, which is what Bankotsu often called Kohaku, much to his annoyance.
External links
Kusarigama on Wikipedia